BABY INCUBATOR NOISE:
CAUSES
Carl A.
AND
SOME
REDUCTION
METHODS
Wales
Submitted in Partial Fulfillment
of the Requirements for the Degree of
Bachelor of Science
at the
Massachusetts Institute of Technology
Signature redacted
Signature of Author..
Department of Mechanical Engineering,
Aoril 15. 1975
Signature redacted
Certified by.........
.......
L
o ..
Signature redacted
..........
Accepted by......
Chairman, Departmental Comlittee on Graduate Students
Archives
(MAR
1 5 1976
4A P
BABY INCUBATOR NOISE:
CAUSES
AND
SOME
REDUCTION
METHODS
by
Carl A.
Wales
Submitted to the Department of Mechanical Engineering on
April 15, 1975 in partial fulfillment of the requirements
for the Degree of Bachelor of Science.
ABSTRACT
An investigation into the internal acoustic noise of
an Isolette Model C-86 infant (baby) incubator is discussed.
The practical lower limit of internal noise is derived.
The internal noise is discussed with regard to its
characteristics,
its sources and its paths.
Methods to
reduce internal noise are set forth and recommendations made
to reduce internal noise to the practical lower limit.
The
major question of whether the incubator noise is harmful or
helpful is not discussed by this author.
D. Graham Holmes
Title:
Asst, Prof., Dept. of Mechanical Engineering
-
2
-
Thesis Advisor:
ACKNOWLEDGEMENTS
The author would like to acknowledge the following:
.
Air Shields, Inc., including Mr. Ridley and
Mr. Weaver for providing the incubator on
which this study was performed.
.
Dr. Graham Holmes, for his supervision.
.
The personnel of the MIT Acoustics and Vibration
Laboratory for their assistance and the use of
the lab's equipment.
.
Prof. Leehey for his advice and guidance.
.
Dr. Cochrane and the personnel of the nursery of
the Boston Hospital for Women.
.
Miss Brenda Raby, R.N.,
for assistance at the
hospital and for answering many medicallyoriented questions.
Robert Gipe and George Foote for their assistance
and advice.
-
3
-
.
TABLE OF CONTENTS
Page
ABSTRACT...............................................
2
ACKNOWLEDGEMENTS.......................................
3
LIST OF FIGURES........................................
5
LIST OF TABLES.........................................
I.
INTRODUCTION.....................................
6
II.
EQUIPMENT:
DETERMINATION OF THE PRACTICAL
LOWER LIMIT ON NOISE.............................
9
III.
EXPERIMENTAL PROCEDURE...........................
12
IV.
RESULTS AND CONCLUSIONS:
DETERMINATION OF
LOWER PRACTICAL LIMIT TO THE INTERNAL NOISE....... 13
V.
EQUIPMENT:
VI.
EXPERIMENTAL PROCEDURE:
VII.
RESULTS AND CONCLUSIONS OF INTERNAL NOISE
ANALYSIS.........................................
21
NOISE ANALYSIS.......................
17
INTERNAL NOISE............ 19
VIII.
EQUIPMENT:
INTERNAL NOISE REDUCTION.............
29
IX.
EXPERIMENTAL PROCEDURE:
NOISE REDUCTION
METHODS..........................................
30
X.
RESULTS OF NOISE REDUCTION WORK..................
31
XI.
OVERALL CONCLUSIONS AND RECOMMENDATIONS............ 34
REFERENCES....................................... 37
APPENDIX 1....................................... 38
-
4
-
BIBLIOGRAPHY..................................... 41
LIST OF FIGURES
Page
1(a)
Equipment and Interconnections for
Measurements in Hospital Nursery............
10
l(b)
Laboratory Analysis of Recorded Noise.......
10
2
Equipment and Interconnections for
Generating the Sound field in the
Transmission Loss Determination.............
3
Hospital Nursery Room Levels..................
14
4
Sound Isolation.............................
15
5
Equipment and Interconnections for
Laboratory Work...............................
18
6
Incubator as Delivered........................
22
6(a)
Noise with Incubator Off....................
23
7
Internal Noise Level Contours on the
Mattress......................................
8
Accelerations of the Motor................... 27
9
0 to 1000 Hz Spectrum with Microphone
Suspended in Mid-Air in the Baby Compartment.......................................
10
35
0 to 1000 Hz Spectrum with the Microphone
36
Al
With Water in Tank............................
39
A2
Without Water in Tank.........................
40
- 5
-
Positioned at the Baby's Head position......
INTRODUCTION
The noise problem in the baby incubator was discussed
by Falk and Woods [1].
Their data (measured at a hospital)
gives the level inside an incubator which was not operating
as 61 dB and 35 dB(A).
They list the levels with the in-
cubator operating as 74.5
1.8 dB and 57.7
3.3 dB(A).
Isolette, a major manufacturer of incubators, was interested
in determining if incubators are, indeed, this noisy,
what the characteristics of the noise are, the sources of
the noise, the paths for noise transmission, and possible
solutions to reduce the internal noise.
In addressing himself to this problem, this author
divided the project into three parts:
1.
Determine the practical lower noise limit.
2.
Measure and analyze the internal noise.
3.
Decide on methods of internal noise reduction.
The determination of the practical lower noise limit gave
a reference upon which to base any efforts in practical
understanding and reducing the noise.
In this regard, the
background level was measured in the Special Care Nursery
at a Boston Hospital.
Using this level and the trans-
mission loss from outside to inside the incubator, which
- 6
-
was measured in the lab, the lowest obtainable level in-
side the incubator could be determined.
Noise reduction
efforts to reduce self noise below this level would be impractical.
With regard to the internal noise level and
characteristics, the internal amplitude levels were
measured with different configurations and conditions inside the incubator.
Narrow-band spectrum analysis was con-
ducted to determine sources of noise as well as propagation paths.
Noise reduction methods consisted of alterations to
the motor mounting and the fan, and interfering with the
propagation paths.
Theoretical methods were considered but
not tried, for the changes necessary to the incubator were
too majcr.
For this project, an Isolette Model C-86 Infant Incubator (See Pictorial 1)
hood
equipped with the intensive care
(and not equipped with iris access ports) was used.
Both the Infant Servo Control and the standard power pack
were used.
The incubator was mounted on the standard cart
- 7
-
which is supplied with the incubator.
ISOLETTE®
INFANT INCUBATOR
MODEL C-86
4
OPERATING
&
loll
MAINTENANCE INSTRUCTIONS
O ISLETTE
AR
A NARCO MEDICAL COMPANY
-
8PICTORIAL 1
II.
EQUIPMENT:
DETERMINATION OF THE PRACTICAL LOWER
LIMIT
ON
NOISE
In the Special Care Nursery of the Boston Hospital
for Women data was taken in real time using a sound level
Approximately twenty minutes of noise was tape
meter.
recorded also for further data reduction in the laboratory.
Figure l(a)
shows the specific equipments and their inter-
connections used in the work at the hospital.
Analysis in the lab of the noise which was tape recorded was done with a narrow band spectrum analyzer and
spectrum averager.
The specifics of the set-up in the lab
are shown in Figure l(b).
Transmission loss was determined
with the equipment shown in Figure 2.
A piston phone with an output of 124 decibels
(re 2 x 10-5 N/M2)
calibration.
(dB)
at 250 Hz was used in all cases for
Corrections for changes in barometric pres-
sure were ignored for being well below the accuracy
( l dB)
for this project.
The reference of 2 x 10-5 N/M
- 9
-
is used for all decibel measurements in this project.
FIGURE 1-A
EQUIPMENT ANT INTERCONNECTIONS FOR MEASUREMENTS IN HOSPITAL NURSERY
.
B&K422
1PISTONPHONE
NURSERY
tB & K 2203
UNLEEMTR
B & K 4145
B & K 2607
NECRA MODEL SJ
B&K 2619
C HER L EA E EMEASURING AMP
Kr
FEDERAL
SCIENTIFIC UA-15A
SPECTRUM ANALYZER
FEDERAL SCIENTIFIC 1015
SPECTRUM AVERAGER
03
HEWLETT-PACKARD
7015
OSCILLOSCOPE
X-Y PLOTTER
LABORATORY ANALYSIS- OF RECORDED NOISE
FIGURE 1-B
vw
w
EQUIPMENT AND INTERCONNECTIONS FOR GENERATING THE SOUND FIELD IN THE TRANSMISSION LOSS DETERMINATION
B & K 1402
RANDOM NOISE GENERATOR
bwMCINTOS' 240
-
ATLAS 60W
HORN SPAKER
POWER AMP
EQUIPMENT AND INTERCONNECTIONS FOR MEASURING THE SOUND INSIDE THE INCUBATOR
B & K 4220
PISTON PHONE
ISOLETTE
MODEL C-86
B & K 4133
MICROPHONE
B & K 2619
PREAMPLIFIER
FEDERAL SCIENTIFIC UA-15A
SPECTRUM ANALYZER
B & K 2607
MEASURING AMP
FEDERAL SCIENTIFIC 1015
SPECTRUM AVERAGER
HEWLETT-PACKARD 7015
X-Y PLOTTER
OSCILLOSCOPE
FIGURE 2
III.
EXPERIMENTAL PROCEDURE
At the Boston Hospital for Women, twenty minutes of
noise was recorded in various rooms in the Special Care
A calibration tone was recorded on the tape at the
Nursery.
beginning and the end of the noise.
Measurements were then
taken using the sound level meter which was calibrated before
the measurements were taken and checked for calibration afterwards.
Readings were taken at various locations in each of
the rooms in the nursery, in one octave bandwidths, linear
full range and A-weighted full range.
(All measurements
were taken to the nearest whole dB.)
For the determination of the transmission loss,
an
experiment was run in the lab (twice to insure valid data).
A noise field was generated in the room using a random noise
generator.
With the field present, but without the incubator,
measurements were taken to check for any significant normal
modes of the room.
The incubator was then placed in the
field and measurements were taken inside the incubator.
In
addition a plot was made of the frequency spectrum both inside and outside the incubator.
-
12
-
the incubator was not running.)
(Note:
for these tests,
IV.
RESULTS AND CONCLUSIONS: DETERMINATION OF LOWER
PRACTICAL
LIMIT
TO
THE
INTERNAL
NOISE
The results of the noise study of the hospital nurSpectrum analysis of the re-
sery are shown in Figure 3.
corded noise confirmed the data taken from the level measurements.
The noise in the hospital for purposes of further
determination efforts was 63 dB(A).
The sound field in the room for the two transmission
loss experiments was 93 dB and 102 dB.
Measured inside the
incubator at the position of the babies head the level was
measured at 76 and 87 dB.
This gives a transmission loss
(TL) of approximately 15 dB.
The spectra of inside and out-
side the incubator as shown in Figure 4 concurs with this
approximate value.
This TL differs from what be theoretical-
ly determined for the plexiglass because of the various air
leaks in the hood.
(Determining the TL using 1 octave band
levels did not differ significantly from using the broadband level as above).
With the field in the hospital of 63 dB(A) and the
TL of the incubator this places the practical lower noise
level inside the incubator at 48 dB(A).
This figure is con-
-
13
-
servative because the hood on the incubator used did not have
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iris ports, the presence of which would decrease the TL
Also, the hospital nursery was in a quiet period when the
measurements were taken (based on conversation with the
nurses on duty).
In addition the Chief of Pediatrics at
that hospital uses monitoring equipment biased by personal
- 16
-
preference towards quieter models.
V.
EQUIPMENT:
A.
A Strobotac (General Radio Model 1531) was used for
NOISE ANALYSIS
determining motor rotation rate.
It was calibrated
with its internal calibration which uses the 60 Hz
power line voltage as a standard.
Laboratory noise work was conducted using the equipment
and interconnections shown in Figure 5.
The wind
screen was used only for the noise contour studies and
was not necessary for other work.
The level recorder
was used for determining long term (one-half to one day
continuous running) variation in noise level.
was done using the piston phone
-
17
-
B.
Calibration
(described earlier).
w
w
w
w
w
w
EQUIPMENT AND INTERCONNECTIONS FOR LABORATORY WORK
PISTONPHONE
B & K 4220
B& K
WINDSCREEN
B & K 2619
PREAMdPLIFIER
B & K 4145 & 4133
MICROPHONES
B & K 2607
MaSURING AMP
ISOLETTE
MODEL C-86
X-Y PLOTTER
EVLRECORDER
Ho
-
FEDERAL SCIENTIFIC UA-15A
SPECTRUM ANALYZER
FIGURE 5
FEDERAL SCIENTIFIC 1015
SPECTRUM AVERAGER
w
w
VI.
EXPERIMENTAL PROCEDURE:
INTERNAL NOISE
The first step in studying the internal noise was
to determine the rotation rate and blade passage rate of the
fan and its motor.
This was done using a strobotac
calibrated against line frequency.
rotation rate checked.
The fan was marked and
Then the fan was removed from the
motor to check the unloaded rotation rate of the motor.
The fan was equipped with twenty blades which multiplied by
the rotation rate gives the blade passage rate.
The second step was to determine the airflow pattern
inside the incubator.
This was done by releasing some
smoke into the plenum chamber underneath the base plate of
the incubator, letting the smoke flow with the circulating
air.
Having taken care of the background aspects, the
basic set-up for noise studies inside the incubator was
established.
The narrowest frequency range which still con-
tained the significant data was determined by continually
narrowing the range of the analyzer until the full spectral
window was filled with data.
Also the number of spectra
to be averaged was determined by trial and error until the
minimum number was found which did not reflect meaningless
-
19
-
fluctuations.
The standard condition for the incubator was without
water and without a baby.
Measurements were taken in a
quiet lab to remove as many external influences as possible.
Using a wind screen the noise level contour was taken inside the incubator to determine the variations which related
to position of the microphone inside the baby compartment.
Modifications were made to the incubator after running
initial tests.
a simulated baby
These modifications included running with
(a baloon of water with 35 ppt salt by
weight), running without a fan on the motor
(but with the
motor running), selection of wet or dry air for the baby
compartment, use of the different power packs, and the
location
of the make-up air tube in the inlet hole to the
fan volute, additional modifications
(and results)
are
listed in the Appendix.
Vibration spectra were taken using an accelerometer
(not calibrated because only relative measurements were
needed) mounted on the motor and on the base plate of the
baby chamber.
For all acoustic measurements the system was
-
20
-
calibrated frequently using the pistonphone.
VII.
RESULTS AND CONCLUSIONS OF INTERNAL NOISE ANALYSIS
The results of studying the spectra and levels collected are:
1.
As seen in Figure 6, the frequencies of all
significant noise are below 1000 Hz.
This low
frequency content allowed most efforts to be
devoted to study of the limited range 0 to
1000 Hz, considerably reducing the amount of
data that needed to be collected and analyzed.
2.
Figure 7 shows the significance of microphone
location.
Because of the normal mode excitation
around 185 Hz, the difference in noise level
between a mode and an antimode.
varies 7 dB(A)
3.
The location of the make-up air tube within the
supply hole to the fan volute can vary the noise
level up to 4 dB(A).
Removal and reinsertion of the power pack may
affect levels up to 4 dB(A).
This is a significant
problem not only in making laboratory data collection more time consuming by the requirement of
taking the average of multiple readings, but also
in making it difficult to predict levels in an
incubator in use and being operated by hospital
-
21
-
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INTERNAL NOISE LEVEL CONTOURS ON THE MATTRESS
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50
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NUMBERS ON THE AXIS ARE CENTIMETERS FROM THE SUPPLY
DUCT ENT) AND FROM THE REAR.
54
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personnel.
5.
Even after relocating electrical components on
the circuit board in the infant servo control
(ISC) power pack to insure isolated motor
mounting, there is a difference between the
standard power pack and the ISC power pack with
the ISC being up to 5 dB(A) noisier.
6.
The presence of the simulated baby had no effect.
Consequently this author feels that all the work
done in the project is applicable to in-use
situations.
7.
The selection of wet or dry (or some combination)
This points away from air-
air had no effect.
born sound paths through the plenum chambers.
Consequently, the sources of the noise were determined
to be:
The spectra and level results are:
1.
All significant noise was below 1000 Hz as seen
in Figure 6.
The location where the microphone was positioned
had an effect up to 7 dB(A)
-
25
-
2.
as seen in Figure 7.
3.
The location of the make-up tube has up to
4 dB(A) affect.
4.
Removal and reinsertion of the power pack has up
to 4 dB(A) affect.
5.
There was a difference between the power packs
with the infant servo control power pack being
up to 5 dB(A) noisier.
6.
The presence of the simulated baby had no effect.
7.
Selection of wet or dry (or some combination)
had no affect.
The sources of the noise were determined to be:
.
Improper fan volute design which includes a
non-concentric air intake into the volute.
.
Improper fan design.
.
The vibration of the motor
(which is shown in
Figure 8).
The paths for sound propagation between source and
baby compartment are primarily structure born as various
alterations and tests could not prove the existence of
airborn path.
In conclusion, the noise level inside the incubator
-
26
-
is high and is caused by various oversights in design.
(A
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design established before noise was considered a
-
28
-
significant parameter).
VIII.
EQUIPMENT:
INTERNAL NOISE REDUCTION
The laboratory equipment for work in noise reduction methods was the same as used (and described) above
for noise analysis.
For practical reasons a sound level meter
(described
previously) was used at the factory to measure levels in-
-
29
-
side the incubators.
IX.
EXPERIMENTAL PROCEDURE:
NOISE REDUCTION METHODS
The procedure for determining noise reduction methods
consisted of installing a possible method and running the
same tests as were run for initial noise analysis.
Re-
sults were compared and additional modifications made and
retested.
To eliminate the differences induced by uncon-
trollable factors such as removal and insertion of the power
pack, the tests were run up to ten times with seemingly
identical set-ups.
The results were then averaged.
After the shock mount
(for the motor)
testing and
testing of the fan alterations the effectiveness of these
methods was tested at the factory using five incubators
which were chosen at random from the production line.
The
internal noise was measured using the sound level meter.
Levels were recorded before modification and after each
modification.
Power packs were rotated through each in-
cubator to show individual characteristics of each incubator
and each power pack, as well as to provide better analysis
-
30
-
of each modification.
X.
RESULTS OF NOISE REDUCTION WORK
Shock mounts for the motor and minor fan modifications
were tried with success.
Two other methods were tried
without success, and some untried theoretical methods were
examined.
The motor vibration contributed large amounts of
noise to the internal noise in the baby compartment.
Through the use of Uniroyal type 301A Shock Mounts
(the
required parameters of the Shock Mounts were determined
through the use of the Barry Mounts Catalogue
(Ref. 2),
the contribution from the motor was lowered to well below
the practical lower noise limit.
What the precise level
was depended on which power pack was used, but both power
packs had levels below 40 dB(A) when operated without the
fans.
Alterations were made to the fan itself in an effort
to reduce the noise it generated.
The motor end of the fan
had stiffening ribs which were working as radial fan blades
and causing a lower pressure stall area around the shaft.
Relief of this pressure either by removal of the ribs or by
drilling holes through the end of the fan reduced the noise
from the fan to make the overall level inside the baby
-
31
-
compartment to below the practical lower limit.
These two reduction methods were tested at the
factory on other incubators with the results listed in
Table 1.
Note, however, that up to 3 dB(A) of the figures
in the table come from measuring the levels at the factory
at the baby's head as opposed to in mid-air where most
laboratory measurements were made.
Removal of the heater proved that the heater reduced
the noise rather than contributed any.
Reducing the airflow reduced the noise significantly
which the author concluded was an added indication that the
fan is stalling in normal operation.
fan seems almost impervious.
Selection of a new
Reference 3 concludes that
backward facing fan blades are the best type for quietest
operation.
Additional theoretical methods of noise reduction
include the use of non-parallel ends to the baby compartment
which should reduce any multiple reflections of sound off
-
32
-
of the ends of the compartment.
TABLE 1
Power Pack A(ISC)
Power Pack B(Standard)
Fan
w/o
w/o
with
w
w/o
w/o
w
w
Shocks
w/o
with
w/o
w
w/o
w
w/o
w
45.33
40.57
56.8
3.37
5.0
Each X,a
based on
1.48
10 measurements
-
33
-
a,
50.64
1.03
51.17 33.25 59.8 46.82
1.34
0.84 0.40
XI.
OVERALL CONCLUSIONS AND RECOMMENDATIONS
The results of the factory trip proved two
important things:
1.
Results are variable from incubator to incubator,
and even on the same incubator from one moment
to the next.
2.
There is the possibility that the incubator in
the laboratory studies was in fact unique and
therefore the noise reduction achieved on it
will not achieve the same level of results as
on other incubators-even those made by the same
factory.
The noise contours indicate notable contributions to
the noise level measured where the microphone is located.
Spectrum analysis and the noise contour plot indicated a
normal mode associated with this problem.
and 10).
(See Figures 9
Possibly alterations to the ducting at the supply
and exhaust ports could change the noise contour pattern,
by reducing the excitation at the normal mode frequency of
approximate 185 Hz.
Further efforts should be devoted to improving the
design of the fan used, matching the fan volute to the fan,
-
34
-
and better designed ports for the circulation system.
0O X
0 TO TIM CM.
3-59.14
.F
I
I
I
F
FIUR
-ari]h ab
0
10O~zapetru
(cmprto fiue1.Fr
F
9
O t
wih mcroponesusendd i
mi
.
oprmn
I
C
.
Il#,
ox 0toyoTHr cm.
xt.uF rLL. & EIS!--LR
359-14
CO.
vii
7
77
-T--
T
:T
.41
......
...
I
....
I
......
I
i
1
I
to 1000:HZ spectrum wtthe microhone positioned at the
-baby s head position.
L
I
I
REFERENCES
1.
S.A. Falk, M.D.,
and N.F.
Woods, R.N., M.N.,
"Hospital
Noise Levels and Potential Hazards".
2.
Catalogue from Barry Mounts.
3.
J.B. Graham,
"How to Estimate Fan Noise", Sound and
-
37
-
Vibration, May 1972.
APPENDIX
1
ADDITIONAL MODIFICATIONS TESTED TO DETERMINE
THE SOURCES OF INTERNAL NOISE
A.
Levels taken with and without the heater coil in place
revealed the presence of the heater reduces the noise
by a slight amount (less than 2 dB(A).)
B.
The presence of water in the wet air chamber reduced
the noise a slight amount in upper frequencies, but did
not affect the major noise content.
(See Figures
Al and A2).
C.
Operation with the filter taken off the rear of the incubator increased the level considerably.
This was not
investigated further as normal operation will never be
conducted without a filter on the incubator.
D.
Isolation of the hood by removing the hinges did not
alter the levels inside the incubator.
Inserting baffles in the ducting in the plenum chamber
did not change the level leading to the conclusion that
the noise was not airborne.
-
38
-
E.
IN
wJ
ID
vJ4i
"x
e
FREQ. RANGE
0 TO
500
NO. OF SPECTRA AVERAGED
center
-.-..
.~
HZ.
32
a
~
-
DB(A)
-----
A4
-
- Irk--4
-
WITH WATER IN TANK
not recorded
MIKE LOCATION
READING
NOTES:
-----
i
7~~
-----_ _...
-
Tw
39 -
FIGURE Al
- t-7-
t
-.
---- -- --...... -..
_7,_
FIGUR
t
T1~
-7
q
-
t
-
f
-----------
-- -- - --
__
10080
----
--
------
Al
- -
7
..
lK
x7
n
FREQ. RANGE
6
to
500
32
center
NO. OF SPECTRA AVERAGED
MIKE LOCATION
not recorded
7
-
-
-
1:
q
71
-
HZ
- -- -
-
-----
without water
READING
7-
w
S--
NOTES:
--
-
-
4-
-
77
-
--
FIGURE
A
40-
-- *
---
w
-t
7
~
- --------.--
*
-
-7
-ce.te
-
-------------
-- -
-7
-- - -
-----------
-- --I----
BIBLIOGRAPHY
ACOUSTIC MEASUREMENT,
Bruel & Kjaer
MECHANICAL VIBRATIONS, Bruel
FUNDAMENTALS OF ACOUSTICS,
& Kjaer
Kinsler & Frey
NOISE REDUCTION, Beranek
NOISE AND VIBRATION CONTROL, Beranek
HANDBOOK OF NOISE MEASUREMENT,
- 41
-
BRUEL & KJAER CATALOGUE
General Radio